Analog circuits, such as filters and oscillators, are increasingly being fabricated as part of integrated circuits ("ICs"). On any particular IC, the frequency response of a circuit is dependent on the values of the capacitors, resistors, transistors, and other circuit components which are used to form the circuit element. Although circuit layouts can be designed very precisely, in practice, the actual size of circuit components across different ICs tends to be highly variable due to cumulative tolerance errors which occur during the manufacturing process. For example, the actual value of a capacitor on an IC can vary by up to .+-.50% from the nominal design value. Further, the performance is also dependent on the absolute temperature at which the circuit is operating. Thus, it is common practice to provide integrated circuits with various performance characteristics that can be adjusted to compensate for manufacturing variances and operating temperature.
In a conventional arrangement, the performance of bandpass filter circuits and other circuits, such as inductor-capacitance ("LC") based circuits, are tuned or otherwise manually adjusted using a digital signal processor ("DSP") which operates by measuring the performance of a representative circuit as it acts on an externally supplied reference signal. For example, in one particular implementation, a multi-stage filter is tuned by first disconnecting one of the filters from the other stages and connecting its input to a DC reference voltage with a predefined and known magnitude. The magnitude of the DC output voltage is then digitized and stored within the DSP. Next, a reference sinusoidal signal is applied to the filter input. The sinusoidal frequency is chosen to be at or close to the desired filter -3 dB frequency and has a peak amplitude which is equal to the magnitude of the reference DC voltage. The power of the filter output is determined and the DSP compares the two measurements and generates a tuning control signal in accordance with predefined data concerning the behavior of the filter at varying tuning signal inputs. (see, Khorramabadi, "Baseband Filters for IS-95 CDMA Receiver Application Featuring Digital Automatic Frequency Tuning", IEEE International Solid-State Circuits Conference, 1996, page 172-173).
Although such tuning techniques are effective, they depend on the presence of specific reference signals which must be generated off-chip to avoid being compromised by manufacturing variations. Thus, additional circuitry external to the IC is required. In addition, such an arrangement uses valuable chip I/O, which may increase both the cost and complexity of the chip packaging and design.
Accordingly, it would be advantageous to provide a tuning circuit which can automatically tune a component, such as a bandpass filter, without requiring an external reference signal. It would also be advantageous if such a circuit can automatically generate a clock signal for use throughout the chip, which clock signal has a frequency tied to the resident frequency of the tuned circuit.